This artist's rendering shows a cross-section of
the ice shell immediately beneath one of Enceladus' geyser-active fractures,
illustrating the physical and thermal structure and the processes ongoing below
and at the surface.
Image Credit:
NASA/JPL-Caltech/Space Science
Institute
Scientists using mission data from NASA’s Cassini spacecraft have identified
101 distinct geysers erupting on Saturn’s icy moon Enceladus. Their analysis
suggests it is possible for liquid water to reach from the moon’s underground
sea all the way to its surface.
These findings, and clues to what powers the geyser eruptions, are presented
in two articles published in the current online edition of the Astronomical
Journal.
Over a period of almost seven years, Cassini’s cameras surveyed the south
polar terrain of the small moon, a unique geological basin renowned for its four
prominent "tiger stripe” fractures and the geysers of tiny icy particles and
water vapor first sighted there nearly 10 years ago. The result of the survey is
a map of 101 geysers, each erupting from one of the tiger stripe fractures, and
the discovery that individual geysers are coincident with small hot spots. These
relationships pointed the way to the geysers’ origin.
After the first sighting of the geysers in 2005, scientists suspected
repeated flexing of Enceladus by Saturn’s tides as the moon orbits the planet
had something to do with their behavior. One suggestion included the
back-and-forth rubbing of opposing walls of the fractures generating frictional
heat that turned ice into geyser-forming vapor and liquid.
Alternate views held that the opening and closing of the fractures allowed
water vapor from below to reach the surface. Before this new study, it was not
clear which process was the dominating influence. Nor was it certain whether
excess heat emitted by Enceladus was everywhere correlated with geyser
activity.
To determine the surface locations of the geysers, researchers employed the
same process of triangulation used historically to survey geological features on
Earth, such as mountains. When the researchers compared the geysers’ locations
with low-resolution maps of thermal emission, it became apparent the greatest
geyser activity coincided with the greatest thermal radiation. Comparisons
between the geysers and tidal stresses revealed similar connections. However,
these correlations alone were insufficient to answer the question, “What
produces what?”
The answer to this mystery came from comparison of the survey results with
high-resolution data collected in 2010 by Cassini’s heat-sensing instruments.
Individual geysers were found to coincide with small-scale hot spots, only a few
dozen feet (or tens of meters) across, which were too small to be produced by
frictional heating, but the right size to be the result of condensation of vapor
on the near-surface walls of the fractures. This immediately implicated the hot
spots as the signature of the geysering process.
“Once we had these results in hand we knew right away heat was not causing
the geysers, but vice versa,” said Carolyn Porco, leader of the Cassini imaging
team from the Space Science Institute in Boulder, Colorado, and lead author of
the first paper. “It also told us the geysers are not a near-surface phenomenon,
but have much deeper roots.”
Thanks to recent analysis of Cassini gravity data, the researchers concluded
the only plausible source of the material forming the geysers is the sea now
known to exist beneath the ice shell. They also found that narrow pathways
through the ice shell can remain open from the sea all the way to the surface,
if filled with liquid water.
In the companion paper, the authors report the brightness of the plume formed
by all the geysers, as seen with Cassini’s high resolution cameras, changes
periodically as Enceladus orbits Saturn. Armed with the conclusion the opening
and closing of the fractures modulates the venting, the authors compared the
observations with the expected venting schedule due to tides.
They found the simplest model of tidal flexing provides a good match for the
brightness variations Cassini observes, but it does not predict the time when
the plume begins to brighten. Some other important effect is present and the
authors considered several in the course of their work.
The Cassini-Huygens mission is a cooperative project of NASA, the European
Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory (JPL)
in Pasadena, California, manages the mission for NASA's Science Mission
Directorate in Washington. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL. The imaging team consists of
scientists from the United States, England, France and Germany. The imaging team
is based at the Space Science Institute.
Additional details, images and an animation are available at:
More information about Cassini is available at:
and
NASA
Guillermo Gonzalo Sánchez Achutegui
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